For control of internal combustion engines, three important control parameters are spark timing (or fuel injection timing in a diesel engine), the exhaust gas recirculation (EGR) rate, and the air/fuel ratio (AFR). Control of spark timing or injection timing affects the timing of the initiation of the combustion process. Control of EGR and AFR affects the speed and duration of the combustion process.
The AFR is often controlled by an exhaust oxygen sensor to produce a desired AFR in a closed loop system. EGR is generally controlled by a combination of different parameters such as exhaust backpressure, engine coolant temperature, engine speed, throttle position or manifold pressure and has proven difficult to control accurately. Spark or injection timing is generally determined from a stored table indexed by engine speed and load parameters which may build in retard in response to a knock, MAP or throttle movement detector.
To implement EGR, a fraction of the exhaust gases are recycled through a control valve from the exhaust to the engine intake system. The recycled exhaust gas is usually mixed with the fresh fuel-air mixture. EGR can be implemented in internal and external configurations. External configurations can be implemented as either a high pressure or loop pressure loop, depending on whether the recirculated exhaust is introduced to the intake post compressor (high pressure loop) or pre-compressor (low pressure loop).
EGR has a long history of use in both diesel and spark-ignited engines for reduction of NOx emissions. It affects combustion in several ways. The combustion is cooled by the presence of exhaust gas, that is, the recirculated exhaust gas absorbs heat. The dilution of the oxygen present in the combustion chamber reduces the production of NOx. Also, if exhaust gas is being recirculated, less air is breathed by the engine, so reducing the amount of exhaust gas produced.
As a relatively simple and low cost technology, EGR can be expected to be widely used on all types of engines. Of particular interest is that EGR can reduce the need for fuel enrichment at high loads in turbocharged engines and thereby improve fuel economy.